Stencil screen coating comprising polyvinyl alcohol, polyvinyl acetate, and a light sensitizing agent



United States Patent M This invention relates generally to stencil screens and, more particularly, to a masking or coating composition used in making stencil screens. This application is a continuation-in-part of application Serial No. 642,669, filed February 27, 1957, now abandoned.

In the past, various gelatinous materials have been used to coat fine mesh screens utilized thereafter in applying decorations to a substrate. The use of such screens is fairly widespread and usually involves the removal of a portion of the gelatinous material at selected areas of the screen, the openings serving as a passageway for the decorating material and defining the area to be decorated in the subsequent application of the decorating material.

The gelatinous materials are, however, inherently limited by reason of their animal origin. Thus, it has been found that these compositions are quite weak physically and are, consequently, short lived under rigorous application treatments. It has also been found that the gelatinous coatings or masking materials are unable to withstand the elevated temperatures of the screen and recorating material which are more frequently used in modern-day technology.

To overcome these shortcomings of the gelatinous type materials there have been adapted synthetic materials which are tougher and are capable of withstanding elevated temperatures. Coating or masking compositions for stencil screens based on a polyvinyl alcohol polymer have found acceptance in this regard due to their more inherent toughness and ability to withstand elevated temperatures. I

Such compositions conveniently include a light sensitizing agent in order that a photographic process may be utilized in forming the pattern or design. The photographic process si-mply involves placing a film (opaque positive) of the desired decoration over the fine mesh screen bearing a dried coating of the light sensitized com position, and exposing the assembly to a light source, whereupon the light contacted portion is rendered water insoluble, while the portions not contacted by light remain water soluble. The thus developed stencil screen is washed, removing the water soluble portion, and thereby yielding the final stencil screen through which the decoration or printing may be applied as, for instance, by a squeegee.

While the above described polyvinyl alcohol polymer based materials are found to be relatively durable and resistant to elevated temperature applications of the decorating materials, they are notably deficient in at least one respect. Thus, it has been found that their use yields a decorated substrate which is lacking in sharpness of definition. Furthermore, decoration in the form of a printed message or identifying name is noticeably fuzzy as to the line detail and therefore, is unattractive and unappealing.

Accordingly, it is an object of this invention to provide an improved coating or masking material for stencil screens used in applying a decoration to a substrate.

It is another object of this invention to provide such a coating or masking material which yields a stencil screen which is tough, durable, and capable of withstanding high temperature application of decorating material, for instance, a vitreous enamel.

3,100,150 Patented Aug. 6, 1963 It is a particular object of this invention to provide a stencil-screen masking or coating composition which yields a stencil screen having sharply defined designs and/or printing, thereby permitting accurate and sharp reproduction of the intended design and/ or printing decoration on a substrate.

It is yet another object of this invention to provide an improved stencil screen which possesses the above characteristics.

In attaining the objects of this invention, one feature resides in forming a stencil screen coating capable of being applied to a stencil screen to fill the voids thereof, with portions of the coating being removed according to photographic methods to yield a preselected pattern of improved line sharpness, said coating consisting essentially of (a) an aqueous solution of a polyvinyl acetate homopolymer, wherein a large percentage of the carboxyl groups are hydrolyzed to OH groups, (b) a light-sensitizing agent and (c) an emulsion of an unhydrolyzed homopolymer of polyvinyl acetate as the film-forming in gredients, the amount of (c) to (a) being from 1:1 to 1:125 parts by weight.

Other objects, features, and advantages of this invention will become evident from the following detailed description of the manner and mode of practicing the invention.

Basically, this invention comprises the discovery that an aqueous solution of a polyvinyl alcohol polymer for coating a stencil screen may be advantageously modified by the inclusion therein of a liquid polyvinyl adhersive. More definitely stated, the stencil screen coatings of the prior art comprising an aqueous solution of a polyvinyl acetate homopolymer wherein a large percentage of the carboxyl groups have been hydrolyzed to the OH groups may be modified so as to obviate the aforesaid disadvantages with respect thereto by incorporating therein from about 1:1 to 1:125 parts by weight of an emulsion of an unhydrolyzed homopolymer of polyvinyl acetate. Since portions of the screen coating will be removed according to photographic methods to.yield a preselected pattern of improved line sharpness on the screen, it is necessary to include a light-sensitive agent in the coating composition to permit the aforesaid photographic method of production of the desired pattern. Polyvinyl alochol polymers suitable for the invention are those sold under the trademark Elvanol by the E. I. du Pont de Nemours & Co., Inc., Wilmington, Delaware, and which are identified as aqueous solutions of polyvinyl acetate homopolymers wherein a sepcific percentage of the carboxyl groups have been hydrolyzed to the OH groups. Thus, various grade designations have been assigned to the Elvanol homopolymers according to the particular percentage of carboxyl groups which have been hydrolyzed to the OH group. These include the following listed compounds.

Composition: Percent hydrolysis of carboxyl groups Elvanol 50-42 86-89 Elvanol 52-22 86-89 Elvanol 54-22 91-93 Elvanol 70-05 98.5-l00 Elvanol 7260 99-100 Thus, while an aqueous solution of a polyvinyl acetate homopolymer wherein from about 86 to of the carboxyl groups are hydrolyzed to OH groups may be satisfactorily utilized in the stencil screen coating of the invention, the best results are obtained when from 88 to 99% of the carboxyl groups are hydrolyzed. Furthermore, applicants have found that mixtures of the homopolymers having different degrees of hydrolysis may be satisfactorily utilized. Another polyvinyl acetate homopolymer which has been found to be satisfactory is Gelvatol 1-60 wherein the carboxyl groups are 99% hydrolyzed.

The liquid polyvinyl adhesive which produces the improved results of the invention when incorporated in the stencil screen coating in an amount of from about 1:1 to 111.25 parts by weight with respect to the polyvinyl acetate homopolymer is better defined as an emulsion of an unhydrolyzed homopolymer of polyvinyl acetate. An example of such an emulsion is that sold under the trademark TBC by the Chemical Division of the Borden Company and identified as Elmers Glue. This adhesive is a liquid emulsion of the polymerized addition product of acetylene, (CHECH), and acetic acid, (CH COOH), e.g., polyvinyl acetate.

Other emulsions of unhydrolyzed homopolymers of polyvinyl acetate which have been used and which have proven satisfactory in the stencil screen coating of the invention include that manufactured and sold under the trademark K-2l and that manufactured and sold under the trademark Elvacet. Thus, polyvinyl acetate emulsions, better defined as emulsions of unhydrolyzed homopolymers of polyvinyl acetate, preferably containing from about 55% to 75% solids, have given satisfactory results in modifying the known polyvinyl alcohol polymer stencil screen coatings.

The following examples are included as being merely illustrative of stencil screen coatings of the invention and are not to be considered as limiting the scope of the invention in any manner.

Example I An aqueous solution of a polyvinyl alcohol polymer having a desirable combination of water solubility, viscosity and film strength was prepared by dissolving in 256 ounces of water (a) 16 ounces of a solid polyvinyl alcohol polymer, identified as Grade 54-22 and marketed under the trademark Elvanol, (b) 16 ounces of a solid polyvinyl polymer identified as Grade 72-60 and marketed under the trademark Elvanol. The solid polymer and water were stirred and heated over a simmer flame for about 2 hours to insure the formation of a true solution. The solution was then introduced into a colloid mixing mill having a clearance of 0.010. After one pass, the material was removed and allowed to cool to room temperature. The solution of the aforesaid Elvanol mixture can also be defined as an aqueous solution of a mixture of polyvinyl acetate homopolymer wherein 92% of the carboxyl groups have been hydrolyzed to the OH groups and of polyvinyl acetate homopolymer wherein 98.5% of the carboxyl groups are hydrolyzed to OH groups. There was next prepared a dye solution containing 4 ounces by volume of a blue paste dye (identified as Monastrol Fast Blue BFP dye) in 32 ounces of water. The dye solution was strained through a 165 mesh wire screen and stirred into the polyvinyl alcohol mill product. To this was added 6 ounces of a concentrated solution (at 72 F.) of potassium dichromate, a light sensitizing agent. Twelve ounces of this solution were combined with an equal weight of a liquid polyvinyl acetate emulsion adhesive known as Elmer's Glue (sold under the trademark TBC, together with ounce of concentrated potassium dichromate solution, yielding one embodiment of the stencil screen masking composition of the invention. The composition varies in color from light blue to medium green, depending upon the concentration of sensitizer, e.g., dichromate, utilized.

The composition, which is a viscous liquid, was then {This grade designates a polymer reported to have a viscosity of 22 seconds as determined by the falling ball method on a 4% (weight) solution of the polymer and water at 68 F and further indicates about a 92 mol percent l1ydrolysis product, calculated from determinations of saponification number (number of milligrams of KOH required to saponify completely one gram of the polymer).

'This grade designates a polymer reported to have a viscosity of 60 seconds and further indicates about a 98.5 mol percent hydrolysis product.

applied to a stainless steel fine mesh screeen while in a vertical position. A squeegee type apparatus was utilized to coat both sides of the fine mesh screen in order to insure complete coverage of the screen and thereby avoiding the presence of any voids therein. The screen coated with the composition was then dried in a darkroom held at a temperature of between 70 F. to F. for about 15 minutes. An opaque positive film, in which the ultimately desired printing or lettering corresponded to the dark area of the film and the surrounding areas were translucent (light transmissible), was placed over the coated and dried screen, and this assembly was then exposed, for about 1 minute, to light issuing from an arc lamp. The light-exposed stencil screen was then given a mild wash whereupon the composition which had not been exposed to light washed away leaving the stencil bearing the desired design or printing outlined by the water insoluble (light contracted) composition remaining on the screen.

Example II 25 grams of a polyvinyl acetate homopolymer having 88% of the carboxyl groups hydrolyzed to the OH groups was dissolved in 200 grams of water. To this was added 1 gram of solid Monastral Blue dye and 10 grams of a concentrated water solution of potassium dichromate light sensitizer. This solution was combined with an equal weight of an emulsion of an unhydrolyzed homopolymer of polyvinyl acetate (Elmers Glue). The resulting solution was applied to a stencil screen and exposed by an arc lamp. The pattern produced on the screen was very clear and sharp.

Example III 25 grams of a polyvinyl acetate homopolymer having 99% of the carboxyl groups hydrolyzed to the OH groups was dissolved in 200 grams of water. To this was added 1 gram of solid Monastral Blue dye and 10 grams of a concentrated water solution of potassium dichromate light sensitizer. This solution was combined with 190 grams of an emulsion of an unhydrolyzed homopolymer of polyvinyl acetate and the resulting composition was applied to a stencil screen and exposed by an arc lamp. The pattern produced on the screen was very clear and sharp.

Example IV 12.5 grams of a polyvinyl acetate homopolymer having 99% of the carboxyl groups hydrolyzed to the OH group and 12.5 grams of a polyvinyl acetate homopolymer having 88% of the carboxyl groups hydrolyzed were dis solved in two hundred grams of water. Io this was added 1 gram of solid Monastral Blue dye and 10 grams of a concentrated water solution of potassium dichromate light sensitizer. This solution was combined with 190 grams of an emulsion of an unhydrolyzed homopolymer of polyvinyl acetate and the resulting composition was applied to a stencil screen and exposed by an arc lamp. A very clear and sharp pattern was produced on the screen.

The stencil screens made in accordance with the invention possess outstanding utility in the application of a vitreous enamel decoration, sometimes referred to as applied color, to a substrate, e.g., glass. Thus it was found that a stencil screen, such as that formed in Example I, satisfactorily endured the application of a hot (200 F.) mixture, of a thermoplastic vehicle and a vitreous enamel without any observable degradation of the masking composition or any deterioration in the quality of the applied color decoloration after repeated use of the screen. A glass bottle bearing a decoloration in the form of printing as applied with the stencil screen described above demonstrates remarkable sharpness of definition and line detail. In contrast, a glass bottle bearing an identical vitreous enamel decoration in the form of printing, and deposited by a stencil screen utilizing a masking composition not including the above referred to polyvinyl acetate emulsion was appreciably fuzzy as to line detail.

Exactly why the composition of this invention yields the desirable results described above is not known and, while it is not intended to be bound by any theory, it is believed that the weakness and indefiniteness of line in the aforesaid screens of the prior art is perhaps caused by the formation of irregular curved lines of the composition between the individual wires of the screen as the composition changes from water soluble to water insoluble in the photographic developing step. Whether this is caused by surface tension or viscosity characteristics or other phenomena is not known. It has, however, been definitely established that the composition of this invention enables the depositing of vitreous enamels, in patterns, designs and/or printing, the line definitions and detail of which are markedly superior to those obtainable heretofore. The compositions of this invention may be applied to stencil screens formed from stainless steel, nylon, silk, or any other conventional materials.

The masking compositions of this invention are easily removed from the stencil screen, permitting reuse theerof. In this regard, it has been found convenient to remove the composition from the metallic wire screens by simply burning the material with a flame. For nylon or silk screens, a solution of bleach, e.g., a hypochlorite is conveniently used.

The light sensitizing agent utilized in forming the composition of this invention may be any of those conventionally used in the art. Essentially, the light sensitizers are oxidizing agents, although it has been found that the dichromates of potassium, sodium and/ or ammonium are preferred.

The amount of light sensitizing agent required is simply the amount which will cause the coating composition to proceed from a stage of water soluble to water insoluble. The above-recited ratio of 4 ounces of dye per 32 ounces of dry polymer was ample for this purpose. It has also been found that as little as 1 ounce of dye per 20 ounces of dry polymer will be sufficient. It must be recognized, of course, that the brilliance of this light source and the time of exposure are factors in this phenomenon and will affect the amount of light sensitizer required.

The function of the dye in the composition of the invention is primarily that of imparting a color to the composition for ease of discernment of the areas of the screen still covered after the washing operation, thus aiding in the application of the decorating materials through the areas defined thereby. The most advantageous results, accomplishing the overall advantages described herein, have been obtained by using a blue or green dye as distinguished from the red, yellow or orange dyes.

While there has been disclosed certain preferred embodiments of this invention, it is not thereby desired or intended to be limited solely :thereto, for the precise proportions of the materials utilized may be varied and equivalent chemicals may be employed, if desired, without departing from the spirit and scope of the invention as defined in the appended claims.

We claim:

l1. A stencil screen coating for use in filling .the voids of a stencil screen and subsequently having portions thereof removed according to photographic methods to form a preselected pattern of voids on said screen, consisting essentially of (a) an aqueous solution of a polyvinyl acetate homopolymer wherein from about 86% to 100% of the carboxyl groups are hydrolyzed to OH groups, (b) a light-sensitizing agent to permit photographic method production of the desired pattern by rendering water-insoluble that portion of said coating exposed to light and (c) an emulsion of an unhydrolyzed homopolymer of polyvinyl acetate as the film forming ingredients, the amount of (c) to (a) being from 1:1 to 111.25 parts by weight.

2. A stencil screen coating for use in filling the voids of a stencil screen and subsequently having portions thereof removed according to photographic methods to form a preselected pattern of voids on said screen, consisting essentially of (a) an aqueous solution of a polyvinyl acetate homopolymer wherein from about 88% to 99% of the carboxyl groups are hydrolyzed to OH groups, (b) a light-sensitizing agent to permit photographic method production of the desired pattern by rendering waterinsoluble that portion of said coating exposed to light and (c) an emulsion of an unhydrolyzed homopolymer of polyvinyl acetate as the film forming ingredients, the amount of (c) to (a) being from 1:1 to 121.25 parts by weight.

3. The stencil screen coating as defined in claim 2 wherein the amount of (c) is equal in parts by weight to the amount of (a).

4. The stencil screen coating as defined in claim 1 wherein said light-sensitizing agent is a dichromate lightsensitizing agent.

5. A stencil screen coating for use in filling the voids of a stencil screen and subsequently having portions thereof removed according to photographic methods, to form a preselected pattern of voids on said screen, consisting essentially of (a) an aqueous solution of a mixture of polyvinyl acetate homopolymers wherein from about 88%-100% of the carboxyl groups are hydrolyzed to OH groups, (b) a dichromate light-sensitizing agent to permit photographic method production of the desired pattern and (c) an emulsion of an unhydrolyzed homopolymer of polyvinyl acetate as the film forming ingredients, the amount of (c) to (a) being from 1:1 to 1:125 parts by Weight.

6. A stencil screen coating for use in filling the voids of a stencil screen and subsequently having portions thereof removed according to photographic methods, to form a preselected pattern of voids on said screen, consisting essentially of (a) an aqueous solution of a mixture of polyvinyl acetate homopolymers wherein from about 88%- 99% of the carboxyl groups are hydrolyzed to OH groups, (b) a wdichromate light-sensitizing agent to permit photographic method production of the desired pattern and (c) an emulsion of an unhydrolyzed homopolymer of polyvinyl acetate as the film forming ingredients, the amount of (c) to (a) being from 1:1 to 121.25 parts by weight.

7. A stencil screen coating capable of :being applied to a stencil screen to fill the voids thereof, and portions of said coating removed according to photographic methods to yield a preselected pattern of improved line sharpness, said coating consisting essentially of (a) an aqueous solution of a mixture of polyvinyl acetate homopolymer wherein 92% of the carboxyl groups have been hydrolyzed to the OH groups and polyvinyl acetate homopolymer wherein 98.5% of the carboxyl groups have 'been hydrolyzed to the OH groups, (b) a dichromate light-sensitizing agent to permit photographic method production of the desired pattern and (c) an emulsion of an unhydrolyzed homopolymer of polyvinyl acetate as the film forming ingredients, the amount of (c) to (a) being from 1:1 to 1:115 parts by weight.

8. A stencil screen coating for use in filling the voids of a stencil screen and subsequently having portions thereof removed according to photographic methods, to form a preselected pattern of voids on said screen, consisting essentially of (a) an aqueous solution of a polyvinyl acetate homopolymer wherein about 92% of the carboxyl groups are hydrolyzed to OH groups, (b) a dichromate lightsensitizing agent to permit photographic method production of the desired pattern and (c) an emulsion of an unhydrolzed homopolymer of polyvinyl acetate as the film forming ingredients, the amount of (c) to (a) being from 1:1 to 1:125 parts by weight.

9. A stencil screen coating for use in filling the voids of a stencil screen and subsequently having portions there of removed according to photographic methods, to form a preselected pattern of voids on said screen, consisting essentially of (a) an aqueous solution of a polyvinyl acetate homopolymer wherein about 88% of the carboxyl groups are hydrolyzed to OH groups, (b) a dichromate light-sensitizing agent to permit photographic method production of the desired pattern and (c) an emulsion of an unhydrolyzed homopolymer of polyvinyl acetate as the film forming ingredients, the amount of (c) to (a) being from 1:1 to 1:125 parts by weight.

10. A stencil screen coating for use in filling the voids of a stencil screen and subsequently having portions thereof removed according to photographic methods, to form a preselected pattern of voids on said screen, consisting essentially of (a) and aqueous solution of a polyvinyl acetate homopolymer wherein about 99% of the carboxyl groups are hydrolyzed to OH groups, (b) a dichromate light-sensitizing agent to permit photographic method production of the desired pattern and (c) an emulsion of an unhydrolyzed homopolymer of polyvinyl acetate as the film forming ingredients, the amount of (c) to (a) being from 1:1 to 1:1.25 parts by Weight.

11. A stencil screen coating capable of being applied to a stencil screen to fill the voids thereof, and portions of said coating removed according to photographic methods to yield a preselected pattern of improved line sharpness, said coating consisting essentially of (a) an aqueous solution of a mixture of polyvinyl acetate homopolymer wherein 88% of the carboxyl groups have been hy- References Cited in the file of this patent UNITED STATES PATENTS 2,110,319 Bell Mar. 8, 1938 2,174,629 Greiner Oct. 3, 1939 2,184,311 Meigs Dec. 26, 1939 2,388,600 Collins et al. Nov. 6, 1945 2,508,341 Wilson May 16, 1950 2,558,548 Eckey June 26, 1951 2,990,281 Printy et al. June 27, 1961 OTHER REFERENCES Du Pont Elvanol, pub. Du Pont Co., Inc., Wilmington, Del., 1947, pages 4 and 23.

Du Pont Elvanol, pub. Du Pont Co., Inc., Wilmington, Del., 1953, pages 4, 5, 21 and 23.

Bovey: The Effects of Ionizing Radiation on Natural and Synthetic Polymers, Interscience Pub., Inc., N.Y., 1958, pages 145-146 and 148. 

1. A STENCIL SCREEN COATING FOR USE IN FILLING THE VOIDS OF A STENCIL SCREEN AND SUBSEQUENTLY HAVING PORTIONS THEREOF REMOVED ACCORDING TO PHOTOGRAPHIC METHODS TO FORM A PRESELECTED PATTERN OF VOIDS ON SAID SCREEN, CONSISTING ESSENTIALLY OF (A) AN AQUEOUS SOLUTION OF A POLYVINYL ACETATE HOMOPOLYMER WHEREIN FROM ABOUT 86% TO 100% OF THE CARBOXYL GROUPS ARE HYDROLYZED TO OH GROUPS, (B) A LIGHT-SENSITIZING AGENT TO PERMIT PHOTOGRAPHIC METHOD PRODUCTION OF THE DESIRED PATTERN BY RENDERING WATER-INSOLUBLE THAT PORTION OF SAID COATING EXPOSED TO LIGHT AND (C) AN EMULSION OF AN UNHYDROLYZED HOMOPOLYMER OF POLYVINYL ACETATE AS THE FILM FORMING INGREDIENTS, THE AMOUNT OF (C) TO (A) BEING FROM 1:1 TO 1:25 PARTS BY WEIGHT. 